Methods for producing a shaped article, which have heretofore been employed, include cutting of a metal from the viewpoint of processability and melt molding of a fiber-reinforced plastic (hereinafter referred to briefly as an "FRP") in a case where the shape of an article to be produced is complicated.
However, a shaped article made of a general-purpose metal is very heavy though it is high in rigidity and strength. The thermal expansion coefficient of such a shaped article, though low as compared with that of a resin molding, is unsatisfactory for use of the article as a part or a member in the field of optical instruments, etc., wherein very high dimensional accuracy is required. On the other hand, the use of a metallic material having a low thermal expansion coefficient involves a problem of high cost which is not economical.
Meanwhile, a shaped article (molding) of a fiber-reinforced plastic is comparatively easily obtained according to melt molding, examples of which molding are a molding of SMC (sheet molding compound), a molding of BMC (bulk molding compound), a molding obtained by stamping molding and a molding obtained by injection molding, in each of which moldings staple fibers are used.
However, such moldings (shaped articles) involve a problem of high thermal expansion coefficient because of difficulties in controlling the fiber volume content (Vf) and the fiber orientation.
In view of this, there have been proposed external pressure molding methods and internal pressure molding methods, wherein various molds are used.
The external pressure molding method, wherein a material to be molded is pressed against an inner mold of a metal or the like by means of an external atmospheric pressure or a shrinkable tape, involves such problems that no dimensional accuracy of external shape of the resulting molding can be secured, and that an additional working operation such as polishing is indispensable. For example, Japanese Patent Publication Gazette No. Hei 3-27375 (27,375/1991) discloses a method for producing a fiber-reinforced plastic molding, wherein continuous fibers are knitted together and wound around the surface of a metal, then impregnated with a matrix, and then molded by applying thereto an external pressure via a rubbery elastic body. According to this method, however, the female mold (outer mold) the rubbery elastic body serving as a female mold (outer mold) has such a degree of freedom as to make the external pressure uneven so that the molding material cannot uniformly be pressed against a male mold (inner mold). As a result, no dimensional accuracy of the external shape of the resulting molding can be secured, so that processing such as polishing or causing macroscopic nonuniformity and local variation in the sectional thickness of the resulting molding have been required. Further, since the rubbery elastic body serving as a female mold (outer mold) absorbs the pressure, it is hard to apply the external pressure to the resulting molding, thus leading to a high void content and hence many void defects in the resulting molding.
On the other hand, internal pressure molding (forming) is a method wherein a material to be molded is pressed against an outer mold of a metal or the like by means of an expansible core. Such internal pressure molding, though capable of producing a molding high in dimensional accuracy of its external shape, raises problems as to difficulties in producing a complicatedly shaped structure and loading a to-be-molded material Into an outer mold, and maybe unsuccessful or uneven application of a pressure to the corner, rib, etc., of a mold may occur, and failure in securing dimensional accuracy of sectional thickness of the resulting molding may result. For example, Japanese Patent Publication Gazette No. Hei 4-22410 (22410/1992) discloses a method of internal pressure molding using a flexible mandrel, which however raises such the above problems.